This verse is from the Bhagavad Gita, Chapter 6, Verse 35. It is written in Sanskrit and translates to English as follows:
śhrī bhagavān uvācha
asanśhayaṁ mahā-bāho mano durnigrahaṁ chalam
abhyāsena tu kaunteya vairāgyeṇa cha gṛihyate || 35 ||
"O mighty-armed one (Arjuna), no doubt it is very difficult to control the mind because it is extremely fickle and unstable. But, it can be controlled by practice and renunciation."
Explanation in detail:
The natural tendency of the mind is fickle and unstable. It is constantly wandering in various thoughts, desires and emotions. Therefore it seems very difficult to control it. But Sri Krishna says that through practice (meditation, sadhana) and detachment (non-attachment to worldly things and illusions) It is possible to control the mind.
This shloka simplifies the process of controlling the mind and attaining enlightenment. And explains in practical terms.
Shri Bhagwanuvach = Shri Bhagwan said.
अन्संश्यम = Without doubt, without any doubt.
Mahabaho = Oh great warrior (Address to the mighty Arjuna).
Mano = mind.
Durnigraham = Difficult to control.
Chalam = fickle, not stable.
Abhyasen = through practice.
Tu = but.
Kaunteya = O son of Kunti (address to Arjuna).
Vairagyaen = Vairagya (renunciation of attachment).
Ch = and.
Grihyate = can be tamed.